Trehalose (.alpha.-glucopyranosyl-.alpha.-D-glucopyranose) is a dimer of glucose molecules linked through their reducing groups. Because of its unusual combination of chemical properties compared to other sugars, including its lack of reducing groups, slow hydrolysis and ability to form a non-deliquescent glass, it is one of the most effective known preservatives of proteins, cellular membranes and other biological compounds in vitro. Also, living organisms that contain large amounts of trehalose are characteristically those often exposed to osmotic, dehydration and heat stresses, such as insects, certain litoral animals and many microorganisms, including yeasts and bacteria. There is circumstantial evidence (summarised by Wiemken [1990] Antonie van Leeuwenhoek 58, 209-217) that the primary role of trehalose in baker's yeast is to confer resistance to these stresses. However, it has also been suggested (Nwaka et al [1994] FEBS Letters 344, 225-228; Van Dijk et al [1995] Applied Environ. Microbiol. 61, 109-115) that the accumulation of trehalose in baker's yeast is not, by itself, sufficient to confer stress-tolerance.
High levels of trehalose occur in the so-called resurrection plants, such as the pteridophyte, Selaginella lepidoqhylla, which can survive prolonged desiccation and heat exposure (reviewed by Avigad [1982] in Encyclopedia of Plant Research (New Series) 13 A, pp 217-347). The great majority of vascular plants, however, are unable to synthesise trehalose. Such plants often accumulate other "compatible" solutes, including glycine betaine, proline and various polyols, in response to stresses such as drought that decrease the availability of intracellular water (reviewed by McCue & Hanson [1990] Trends in Biotechnology 8, 358-362).
There are very few reports of trehalose in angiosperms, and these usually describe small amounts that could reflect microbial contamination (e.g., Kandler & Senser [1965] Z. Pflanzenphysiol. 53, 157-161; Oesch & Meier [1967] Phytochemistry 6, 1147-1148). Indeed, it has been suggested that trehalose is toxic to many plant tissues (Veluthambi et al. [1981] Plant Physiol. 68, 1369-1374), especially those with little or no trehalase activity (trehalase is the enzyme that converts trehalose to glucose). However, at least one angiosperm, Myrothamnus flabellifolia (another "resurrection" plant), accumulates significant amounts of trehalose (Bianchi et al. [1993] Physiologia Plantarum 87, 223-226), showing that there is not an absolute compatability barrier between trehalose and angiosperms.
The absence of trehalose from most angiosperms and reported toxicity in some suggests that introduction of a trehalose synthetic pathway into these plants might sometimes have deleterious effects. On the other hand, successful production of trehalose in plants would have substantial advantages. Trehalose accumulated in, e.g., the storage organs of sugar beet, potato, onion etc, could be commercially extracted to provide trehalose at costs that make it competitive with sucrose in certain applications. These applications include the manufacture of dried foods (milk and egg powders, soups, fruit purees, etc), because trehalose preserves the flavour and texture of many food stuffs through economically attractive drying procedures, and is much superior in this regard to sucrose (see, e.g., Roser [1991] Trends in Food Science & Technology, July issue, pp. 166-169; Roser & Colaqo [1993] New Scientist, May issue, pp. 25-28). Compared to sucrose, the non-sweetness of trehalose is a further advantage in many cases (soups, egg powders), as is the fact that it does not yield fructose, which is perceived as a health risk. However, the high price of trehalose makes its use in the dried food industry prohibitively expensive. Secondly, production of trehalose in the edible portion of certain plants could extend the shelf life of products such as tomatoes. Thirdly, accumulation of trehalose in sensitive tissues could increase the tolerance of plants towards frost, drought, high salinity and similar stresses.